When an optical apparatus, such as a digital still camera or a scanner, generates a digital image, noise occurs in color components and, as a result, the image has roughness. In recent years, an optical device has also been mounted on a mobile product, such as a cellular phone, as a camera module, and has been used to provide a low-cost, small-area color noise filter circuit. With the progress of increasing the number of pixels for the camera module, a light amount and area per pixel have been decreasing.
It is known that color noise is more likely to occur when luminance to a subject is low, so it is presumable that a problem of color noise progressively occurs in the camera module. As such a low-cost, small-area color noise filter may provide benefit.
It is conceivable that a method of reducing color noise includes applying a typical filter, such as a low-pass filter, an ε filter, a median filter and a Laplacian filter. In order to optimally reduce color noise, it is conceivable that low-pass filter process or median filter process is carried out on pixels of about vertical 5 pixels by horizontal 5 pixels.
However, in terms of circuit implementation, a large memory space of about at least five lines in an image frame is required, so it is inappropriate for a mobile device, such as a recent cellular phone that requires low cost and small area.
In the existing art, Japanese Unexamined Patent Application Publication No. 2003-259126 discusses an image processing apparatus that reduces noise of an image and suppresses degradation of the image. Japanese Unexamined Patent Application Publication No. 2003-123063 discusses an image processing apparatus that is able to detect a color edge with a simple configuration and that carries out color noise reduction while preventing color bleeding.
An example of reducing color noise by executing low-pass filter process on pixels of about vertical 5 pixels by horizontal 5 pixels is illustrated in FIG. 1 and FIG. 2. A method is discussed in which, when the center of a block of vertical 5 pixels by horizontal 5 pixels is set as a target pixel, a mixture of peripheral pixels at a given rate is used as a color difference of the target pixel in the low-pass filter. However, as illustrated in FIG. 2, it is necessary to have a memory space for holding data corresponding to the necessary number of lines. If it is assumed that color noise filter is applied only in the horizontal direction of an image frame, there is no problem in terms of circuit implementation for applying a low-pass filter, an ε filter, a median filter, or the like, as long as buffers of the necessary number of pixels are present in the horizontal direction. However, when the color noise filter is applied only to pixels in the horizontal direction, the stripe of a color difference signal occurs in the horizontal direction. Thus, it is necessary to provide a method of executing color noise filter using pixels in the vertical direction. Applying the method described above, in order to carry out color noise filter process using the pixels in the vertical direction, it would be necessary to prepare line buffers of the necessary number of lines in the vertical direction. Therefore, a memory space for holding data of the necessary number of lines is required. Thus, cost and area problematically would increase in a mobile device, such as recent cellular phones, that requires low cost and small area.